The present invention relates to a nonconductive extension for mounting an encoder on a shaft of an electric motor.
Current electronic controls for elevator system motors operate in a closed loop that requires a velocity feedback signal. When existing elevator equipment is modernized with these electronic controls, the previously installed electric traction motor must be retrofitted with a velocity feedback transducer. A rotating shaft of the traction motor must be fitted with a stub shaft that turns a transducer shaft through a coupling. The stub shaft is inserted into a hole drilled into the end of the motor shaft. If the stub shaft does not run true, it is tapped with a hammer to get it close. An adapter is mounted to the motor frame to support a transducer, such as a shaft encoder. A shaft of the encoder is attached to the stub shaft using a flex coupling. Installing the stub shaft on a rotational center of the output shaft is very difficult and time intensive. Also, any misalignment could cause a false signal feedback and premature coupling failure.
The German patent DE 2 312 547 shows a shaft mounted tachometer provided with a toothed ring coupled to the shaft by a spring and mounted to the shaft by a needle bearing. The rotating teeth cooperate with a stationary yoke and an induction coil to form a magnetic circuit generating a speed dependent signal.
The present invention concerns an apparatus and method for adding a velocity transducer to an electric motor such as an elevator drive motor. A shaft extension is mounted on an end surface of a motor shaft by drilling and tapping a bore at the shaft center as parallel as possible. A fastener with a bevel washer between a pair of flat washers attaches the extension to the motor shaft. With the fastener assembly snug, the extension is trued radially with a dial indicator. The bevel washer functions as a spring lock and distributes any off center clamping force produced from non-concentric hole drilling. A shoulder on the extension abuts the motor shaft end surface to maintain parallelism of the longitudinal axes of the shaft extension and the motor shaft. The shaft extension has a plastic construction to electrically isolate the transducer from the drive equipment as recommended by the drive equipment manufacturer.
The transducer is a hollow shaft encoder that tightens to the shaft extension by a split collar or a taper lock. An anti-torque device secures the encoder housing to the motor frame. The flat washers and the bevel washer lock the fastener in place and, if the threads in the bore in the motor shaft (which are drilled and tapped in the field) are not concentric with the motor shaft longitudinal axis, the load from the fastener head is evenly distributed to the shaft extension.
The apparatus according to the present invention will save approximately 50% installation and adjustment time, and eliminate callbacks due to broken or loose couplings.
It is an object of the present invention to reduce installation and adjustment labor for velocity transducers, and to eliminate service callbacks due to a coupling malfunction.